I am confused about the difference between internal energy and enthalpy!?
Enthalpy seems redundant to me. The equation is H= U + PV.
Meanwhile, U is internal energy which can only be altered by heat or work. The PV term in enthalpy is the work that is done at constant pressure. I don't understand why PV is added to the internal energy when internal energy already consists of heat and work. PV is a form of work! It is already part of the internal energy so why add it again? It just doesn't add up for me.
Answer:
It's because you need to account for changes to the system and the U includes past work done, not current work. The idea of U and the utility of H become clearer when you think about changes in systems rather that static situations. In that sense, U incorporates all changes in system energy done by PV work *before* the change you are considering. Internal energy can't really be measured anyway, the differential form of the enthalpy relation you have written becomes more important.
Google "thermodynamics enthalpy" and there are a few hits that go into more, and better, detail on what I've written above.
the other answer is correct
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Meanwhile, U is internal energy which can only be altered by heat or work. The PV term in enthalpy is the work that is done at constant pressure. I don't understand why PV is added to the internal energy when internal energy already consists of heat and work. PV is a form of work! It is already part of the internal energy so why add it again? It just doesn't add up for me.
Answer:
It's because you need to account for changes to the system and the U includes past work done, not current work. The idea of U and the utility of H become clearer when you think about changes in systems rather that static situations. In that sense, U incorporates all changes in system energy done by PV work *before* the change you are considering. Internal energy can't really be measured anyway, the differential form of the enthalpy relation you have written becomes more important.
Google "thermodynamics enthalpy" and there are a few hits that go into more, and better, detail on what I've written above.
the other answer is correct
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